The major objective of this proposed research is to understand how growth-promoting peptides initiate cell proliferation. In particular, to determine the role a growth promoting peptide found in serum plays in regulation of growth of mammary gland epithelium. The project involves studies on purification of mammary serum factor, binding of the factor to mammary epithelium of animals in different physiological states, determination of its similarities to other growth promoting peptides and studies in vivo on its effcts on normal mammary gland growth.